Injury to the central nervous system (CNS) can have devastating consequences due to the poor regenerative capacity of neurons in that environment. This contrasts markedly with the comparatively good regenerative capacity of neurons in the peripheral nervous system. See, for example, Homer & Gage, Nature 2000, 407:963–970. Numerous diseases, such as Alzheimer's disease, Parkinson's disease, stroke, head and spinal cord trauma to name a few, are all associated with damage to the CNS that is often severe, even debilitating, long lasting or even permanent. No cure is presently available for these conditions, and even palliative treatments are lacking.
3.1. Neurotrophins
It is now understood that the growth and regeneration of neurons is regulated at least in part by certain polypeptide growth factors, known as neuroptrophins or “NTs,” which bind to and activate cell surface receptors having an intrinsic tyrosine kinase activity. Upon neurotrophin binding, these receptors are believed become autophosphorylated on one or more amino acid residues and subsequently associate with intracellular molecules important for signal transduction. For a review, see Ulrich & Schlessinger, Cell 1990, 61:203–212.
The first identified neurotrophin is known in the art as nerve growth factor (NGF) and has a prominent effect on developing sensory and sympathetic neurons of the peripheral nervous system. See, Levi-Montalcini & Angeletti, Physiol. Rev. 1968, 48:534–569; Thoenen et al., Rev. Physiol. Biochem. Pharmacol. 1987, 109:145–178; Thoenen & Barde, Physiol. Rev. 1980, 60:1284–1325; Whittemore & Seiger, Brain Res. 1987, 434:439–464; Angeletti & Bradshaw, Proc. Natl. Acad. Sci. U.S.A. 1971, 68:2417–2420; Angeletti et al., Biochemistry 1973, 12:100–115. NGF orthologs have also been isolated and characterized in a number of other species, including mice, birds, reptiles and fishes (Scott et al., Nature 1983, 302:538–540; Schwartz et al., J. Neurochem. 1989, 52:1203–1209; and Hallböök et al., Neuron 1991, 6:845–858.
A number of other NTs are also known in the art. These include brain-derived neurotrophic factor (BDNF), which is also known as neurotrophin-2 (NT-2). See, Leibrock et al., Nature 1989, 341:149–152. Still other NTs include a factor originally called neuronal factor (NF) and now commonly referred to as neurotrophin-3 or “NT-3” (Emfors et al., Proc. Natl. Acad. Sci. U.S.A. 1990, 87:5454–5458; Höhn et al., Nature 1990, 344:339; Maisonpierre et al., Science 1990, 247:1446; Rosenthal et al., Neuron 1990, 4:767; Jones & Reichardt, Proc. Natl. Acad. Sci. U.S.A. 1990, 87:8060–8064; and Kaisho et al., FEBS Lett. 1990, 266:187). Neurotrophins-4 and -5 (NT-4 and NT-5) are also known. See, Hallbook et al., Neuron 1991, 6:845–858; Berkmeier et al., Neuron 1991, 7:857–866; Ip et al., Proc. Natl. Acad. Sci. U.S.A. 1992, 89:3060–3064. See also, U.S. Pat. No. 5,364,769 issued Nov. 15, 1994 to Rosenthal. Because it was subsequently seen to be a mammalian ortholog of the Xenopus NT-4 described by Hallbrook et al., supra, the mammalian NT-5 molecule described by Berkmeier et al., supra, is also commonly referred to as NT-4/5. An alignment of NT's BDNF, NT4, NT3, and NGF is provided in FIG. 1.
3.2. Trk Receptors
Neurotrophins mediate their effect through a family of receptor tyrosine kinases that are expressed on the surface of neuronal cells and referred to collectively as Trk-receptors. At least three different Trk-receptors are known and have been described in the art: TrkA, TrkB and TrkC. For a review, see U.S. Pat. Nos. 5,844,092; 5,877,016; 6,025,166; 6,027,927; and 6,153,189 all by Presta et al. Although the structure and sequences of the different Trk-receptors are similar, alternate splicing increases the complexity of this family giving rise to several different isoforms of each receptor. An alignment of the different Trk-receptor amino acid sequences is provided here at FIG. 2A–2C setting forth the consensus sequences and boundaries for the various domains of each receptor. See also, FIGS. 16A–16C in U.S. Pat. No. 5,877,016.
Each of the different Trk-receptors exhibits particular binding affinity for the different neurotrophins, although there is some overlap. Hence, TrkA is believed to bind not only NGF, but also NT-3 and NT-4/5 (but not BDNF). TrkB is believed to bind BDNF, NT-3, NT-4 and NT-4/5, but not NGF. By contrast, TrkC is believed to bind only NT-3 and not any of the other neurotrophins.
A number of studies have validated the Trk-receptors as therapeutic targets for brain repair. See, for example, Liu et al., J. Neurosci. 1999, 19:4370–4387; Menei et al., Eur. J. Neurosci. 1998, 10:607–621; and Kobayashi et al., J. Neurosci. 1997, 17:9583–9595. The Trk-receptors and their ligands have also been studied using X-ray crystallography to obtain three-dimensional structures of the ligand-receptor binding complexes. Wiesmann et al., Nature 1999, 401:184–188; Banfield et al., Structure (Camb) 2001, 9:1191–1199. These and other studies suggest that neurotrophin binding to the Trk-receptors induces dimerization of receptor monomers, resulting in an increase of the receptors' intrinsic tyrosine kinase activity. This increased activity triggers, in turn, signaling cascades that are believed to be beneficial to neurons by promoting neuronal survival, axonal growth, and synaptic plasticity. Snider, Cell 1994, 77:627–638; Kaplan & Miller, Curr. Opin. Neurobiol. 2000, 10:381–391.
There has therefore been considerable recognition that therapeutic compounds which target and activate Trk-receptors (i.e., Trk-receptor “agonists”) would be beneficial and desirable. See, for example, Lindsay et al., Exp. Neurol. 1993, 124:103–118; Olson, Neurochem. Int. 1994, 25:1–3. Moreover, increased levels of certain neurotrophins (e.g., BDNF) are also associated with medical conditions such as epilepsy (Binder et al., Trends Neurosci. 2001, 24:47–53). Hence, even compounds that inhibit Trk-receptor activity (i.e., Trk-receptor “antagonists”) would be beneficial. Despite this long felt need, such compounds have been elusive at best. As large-molecules, the therapeutic delivery of effective levels of neurotrophins themselves presents considerable, possibly insurmountable, challenges. Moreover, natural neurotrophins may interact with other receptors, such as the p75 receptor in neurons, which is associated with neuronal apoptosis and growth cone collapse. Lee et al., Curr. Opin. Neurobiol. 2001, 11:281–286.
However, previous efforts to design peptidomimetic agonists and/or antagonists of Trk-receptors have also been unsuccessful. For example, cyclic peptides derived from loop 1 of the neurotrophin NGF have been reported to moderately mimic the survival activity of NGF. However, these peptides appear to function in a p75, rather Trk-receptor, dependent manner. Long et al., J. Neurosci. Res. 1997, 48:1–17. Some NGF loop 4 cyclic peptides are said to show NGF-like survival activity that is blocked by a Trk antagonist. However, the maximal survival response induced by those peptides is reported to be only 10–15% of the maximal response promoted by the NGF neurotrophin itself. See, Xie et al., J. Biol. Chem. 2000, 275:29868–29874; and Maliartchouk et al., J. Biol. Chem. 2000, 275:9946–9956. Bicyclic and tricyclic dimeric versions of BDNF loop 2 peptides have been shown to have BDNF-like activity. Again, however, the maximal survival response they induce is reported to be only 30% of the maximal response promoted by the natural neurotrophin. O'Leary et al., J. Biol. Chem. 2003, 278:25738–25744 (Electronic publication May 2, 2003).
There continues to exist, therefore, a long felt need for compositions that can modulate (i.e., increase or inhibit) neuronal growth and recovery. There also exists a need for processes and methods (including therapeutic methods) that effectively modulate neuronal growth and recovery.
3.21. The p75 Receptor Neurotrophin Receptor
The p75 receptor is known to play roles in signaling complexes for neuronal apoptosis and growth inhibition. Barker, Neuron 2004, 42:529–533. The p75 receptor is a member of the tumor necrosis factor (TNR) superfamily and is characterized by cysteine-rich domains (CRDs) in its extracellular portion. These CRDs are required for neurotrophin binding, and p75 receptor serves as a low affinity receptor for neurotrophins such as NGF, BDNF, NT-3, and NT-4. Huang and Reichardt, Annu. Rev. Biochem. 2003, 72:609–642. NGF, BDNF, NT-3 and NT-4 can effectively compete with each other for binding to p75 receptor. In inhibitory environments, these neurotrophins can be used to compete out each other's binding to p75 receptor in order to reveal responses that depend solely on Trk signaling. Barker and Shooter, Neuron 1994, 13:203–215.
3.22. The p75 Receptor and the NGF TDIKGKE Motif
It is known that the TDIKGKE (SEQ ID NO:42) motif that constitutes the first β hairpin loop of NGF plays a crucial role in the binding of NGF to the p75 receptor. He and Garcia, Science 2004, 304:870–875; Ibanez et al., Cell 1992, 69:329–341. Furthermore, constrained TDIKGKE (SEQ ID NO:42) motifs interact with the p75 receptor and are expected to compete for neurotrophin binding to this receptor. Longo et al., J. Neurosci. Res. 1997, 48:1–17.
The cyclic peptides and peptidomimetic compounds derived from loop 1 of NGF have been reported to moderately mimic NGF's neuron growth-promoting activity (see U.S. Pat. No. 6,017,878 to Saragovi et al.), and these peptides appear to function in a p75 receptor-dependent manner (Longo et al., J. Neurosci. Res. 1997, 48:1–17). Some NGF loop 4 cyclic peptides are said to show NGF-like neuron growth promotion that is blocked by a Trk antagonist. However, the maximal response induced by those peptides is reported to be only 10–15% of the maximal response promoted by the NGF neurotrophin itself. See Xie et al., J. Biol. Chem. 2000, 275:29868–29874; and Maliartchouk et al., J. Biol. Chem. 2000, 275:9946–9956. Bicyclic and tricyclic dimeric versions of BDNF loop 2 peptides have been shown to have BDNF-like activity. Again, however, the maximal response they induce is reported to be only 30% of the maximal response promoted by the natural neurotrophin. O'Leary et al., J. Biol. Chem. 2003, 278:25738–25744 (Electronic publication May 2, 2003).
3.3. Inhibitory Signals
The central nervous system's limited ability to repair injuries is thought to be at least partly due to the presence of inhibitory products that prevent axonal regeneration—including inhibitors associated with damaged myelin (Berry, Bibl. Anat. 1982, 23:1–11). Indeed, biochemical studies on central myelin have identified two protein fractions that contain inhibitory activity for cell spreading (Caroni & Schwab, J. Cell Biol. 1988, 106:1281–1288) and monoclonal antibodies that bind to those fractions enhance the growth of cultured sensory and sympathetic neurons in what are otherwise non-permissive substrates for neurite growth (Caroni & Schwab, Neuron 1988, 1:85–96). Studies with these same antibodies in lesioned animals have also shown that functional recovery can be obtained by blocking the function of inhibitory molecules associated with myelin (Bregman et al., Nature 1995, 378:498–501; Schnell & Schwab, Nature 1990, 343:269–272). A much more robust regeneration response has been obtained in mice immunized with whole myelin (Huang et al., 1999) further demonstrating that CNS recovery and repair can be enhanced in vivo, by blocking inhibitory factors.
At least three myelin derived molecules are known that are potent inhibitors of axonal growth: the myelin-associated glycoprotein, which is also referred to as “MAG” (described by McKerracher et al., Neuron 1994, 13:805–811; and by Mukhopadhyay et al., Neuron 1994, 13:757–767); Nogo-A (see, Chen et al., Nature 2000, 403:434–439; GrandPre et al., Nature 2000, 403:439–444; and Prinjha et al., Nature 2000, 403:383–384) and the oligodendrocyte myelin glycoprotein (Wang et al., Nature 2002, 417:941–944). The Nogo receptor (also referred to as “NgR”), the ganglioside GT1b and the p75 neurotrophin receptor (also referred to as “p75NTR” or “p75NTR”) have been implicated in mediating responses to all three of these inhibitory molecules. Specifically, binding to NgR is said to be required for inhibitory activity by all three inhibitors MAG, Nogo-A and oligodendrocyte glycoprotein (Domeniconi et al., 2002; Liu et al., 2002; Wang et al., 2002b). However, MAG can also bind directly to the GT1b receptor (Vyas & Schnaar, Biochimie 2001, 83:677–682). Moreover, antibody induced clustering of GT1b receptor can mimic the inhibitory response produced by MAG (see, Vinson et al, J. Biol. Chem. 2001, 276:20280–20285; and Vyas et al., Proc. Natl. Acad. Sci. U.S.A. 2002, 99:8412–8417).
The p75 receptor is the signaling component of a multimeric receptor complex than can bind all three myelin receptors. See Domeniconi et al., Neuron 2002, 35:283–290; Liu et al., Science 2002, 297:1190–1993; Wang et al., Nature 2002, 417:941–944. Interactions between the GT1b and p75NTR receptors have been reported (Yamashita et al., 2002), as have interactions between the NgR and p75NTR receptors (see, Wang et al., 2002a; and Wong et al., 2002). Such interactions with p75NTR are thought to be important in the transmission of inhibitory signals (e.g., from MAG, Nogo-A and/or oligodendrocyte glycoprotein) across the cell membrane. For example, interactions of MAG or a Nogo-A peptide with cells that express NgR increases association of p75NTR with Rho-GDI, and induces the release of RhoA from that complex (Yamashita & Tohyma, Nat. Neurosci. 2003, 6:461–471). This step is a pre-requisite for activation of RhoA and inhibition of growth (Id), and the inhibition of RhoA and/or Rho kinase (a downstream effector of RhoA) effectively circumvents inhibitory activity, e.g., of myelin in cultured neurons (see, for example, Dergham et al., J. Neurosci. 2002, 22:6570–6577; Fournier et al., J. Neurosci. 2003, 23:1416–1423; and Lehmann et al., J. Neurosci. 1999, 19:7537–7547).
As noted above, the various neurotrophins (e.g., NGF, BDNF, NT-3 and NT-4/5) do have dramatic effects on neuronal survival and axonal growth during development. It has been recently suggested that neurotrophins and inhibitory molecules (for example, MAG, Nogo-A and oligodendrocyte glycoprotein) may have an opposing effect on the coupling of p75NTR receptor to Rho-GDI (see, Yamashita & Tohyama, Nat. Neurosci. 2003, 6:461–467). Nevertheless, it has not as of yet been possible to promote robust, long range axonal regeneration using neurotrophins. This is believed to be at least partly due to the inability of neurotrophins to effectively counteract inhibitory signals such as those described above. For example, the treatment of cultured neurons with neurotrophins such as NGF, BDNF or GDNF (glial derived neurotrophic factor) does not normally counteract the inhibitory activity of myelin unless the neurons are first “primed” by exposure to the neurotrophin for several hours before exposure to the inhibitory signal (Cai et al., Neuron 1999, 22:89–101). Such priming, however, is of limited effect, time consuming, cumbersome to apply, and impractical for clinical and other in vivo applications. Moreover (and as noted above), the therapeutic delivery of neurotrophins themselves, which are large molecules, presents considerable and possibly insurmountable technical challenges. Furthermore, neurotrophins may be compromised in their ability to promote regeneration because they bind to the inhibitory complex through their interaction with the p75 receptor. Neurotrophins, which are bound to p75 receptor, cannot activate Trk receptors to overcome inhibitory signaling and to promote neuronal growth.
Hence, there additionally exists a need for compounds that can effectively modulate the effects of inhibitory signals on neuronal growth and recovery—including compounds that effectively modulate effects of inhibitory signals such as those produced by MAG, Nogo-A, oligodendrocyte glycoprotein, NgR, GT1b, p75NTR and/or downstream effectors of these signaling molecules. In particular, there exists a need for compounds that can effectively counteract such inhibitory signals, and/or stimulate neuronal growth and recovery. There also exists a need for processes and methods (including therapeutic methods) that modulate effects of such inhibitory signals and, in particular, for processes and methods that counteract such inhibitory signals and/or stimulate neuronal growth and recovery.
The citation and/or discussion of a reference in this section and throughout the specification is provided merely to clarify the description of the present invention and is not an admission that any such reference is “prior art” to the invention described herein.